1. Field of the Invention
The present invention relates to techniques for communicating data through a communication channel. More specifically, the present invention relates to a method and apparatus for compensating for frequency dependent losses when transmitting signals through a lossy communication channel.
2. Related Art
Advances in semiconductor fabrication technology presently make it possible to integrate large-scale systems, including tens of millions of transistors, into a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip enables increases in the frequency at which such systems can operate, because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays.
However, as the frequency of these systems increases, the communication channels used to transfer data between system components is rapidly becoming a bottleneck. At higher frequencies, a communication channel tends to attenuate the transmitted signal. Consequently, if the system transmits data through the communication channel at a sufficiently high frequency, data can be lost.
System designers often use voltage-mode drivers to transmit data through communication channels. In order to overcome the frequency dependent signal attenuation problem, some voltage-mode drivers perform a “pre-compensation” operation for higher frequency events to compensate for signal loss. This is accomplished by temporarily boosting the drive strength for high frequency events. Unfortunately, boosting the drive strength in a voltage-mode driver also involves decreasing the source resistance of the driver, which can cause an impedance mismatch with the characteristic impedance of the communication channel.
Voltage-mode drivers typically have a drive-strength which is inversely proportional to the source resistance. Therefore, once the source resistance of the voltage-mode driver is set to match the impedance of the communication channel, the drive-strength of the voltage-mode driver is fixed. Hence, a voltage-mode driver cannot compensate for these frequency-dependent losses without causing a corresponding impedance mismatch.
One solution to this problem is to use a current-mode driver which has a source resistance that matches the line impedance of the communication channel. The drive strength of a current-mode driver can be boosted by increasing the current, without changing the source resistance. Unfortunately, a current-mode driver uses significantly more power than the voltage-mode driver, which makes such drivers impractical for many applications.
Hence, what is needed is a method and an apparatus for increasing the data transfer rate through a communication channel without the problems described above.